The invention relates in general to the selective coordination between two or more circuit protective devices upon the occurrence of a fault, and more particularly, to a module for providing zone selective interlock features between interconnected static trip circuit breakers.
U.S. Pat. No. 4,245,318 to Eckart et al., assigned to the common assignee of the instant invention, describes the relationship between time and current for static trip circuit protective devices. This patent is incorporated herein for purposes of reference.
Manufacturers of circuit protective devices, e.g. circuit breakers, fuses, overload relays, etc., provide log-log coordinate graphs of the response time versus current curves for their devices. In the case of a circuit breaker, for example, its response time versus current curve, typically referred to as a trip-time curve, indicates the time in which the breaker will trip in response to a particular level of overcurrent. Thus, if the overcurrent is of sustained overload proportions, the circuit breaker's trip-time curve shows that it will trip within a prescribed time determined by the overcurrent magnitude. Overload tripping is typically performed on an inverse-time basis, i.e. the larger the over-load current, the less time to trip, and vice versa. Circuit breakers are designed to trip on this basis so as not to trip in response to normal momentary overload currents which flow, for example, during motor starting. Overload current responses of fuses and overload relays are also predicated on an inverse-time basis. Circuit protective devices, including circuit breakers, are also designed to respond to overcurrents of short circuit proportions, e.g., ten times rated current, on an instantaneous basis, that is, without intentional delay. A typical circuit breaker clearing time, i.e., the maximum time taken by the breaker to physically open its contacts and interrupt short circuit currents, is typically twenty-five to thirty milliseconds.
Intermediate to the overload and short circuit overcurrent ranges is a heavy overload current range, e.g., three to ten times rated current, which is typically handled on a fixed time delay basis. That is, circuit interruption in response to heavy overload current levels is effected upon the expiration of a predetermined fixed short time delay.
Once an electrical power distribution system has been designed, its loads and operating conditions defined, and the voltages and electrical equipment selected, it remains to determine the appropriate circuit protective devices to be used. The primary concern in this determination is protection of the distribution system in the event of a fault precipitating abnormal overcurrent condition, i.e., to prevent or at least minimize damage to the system, including its conductors and connected loads. Such a fault may be caused by equipment failure, human error, or emergencies of natural origin. Typically, such faults are unpredictable, and thus the selected circuit protective devices must function automatically to fully protect the system, and incidentally to protect personnel and property against the consequential hazards of such faults.
Another principal concern associated with the determination of the circuit protective devices to be used is to minimize the extent and duration of electrical service interruption in the event of a fault. In all but the simplest systems, there are two or more circuit protective devices between a fault and the source of the fault current. In order to minimize electrical service interruption these protective devices should be selective in response such that the one nearest the fault will first attempt to interrupt the fault current. If, for any reason, this protective devices does not clear the fault in timely fashion, the next upstream protective device will attempt to do so, and so on. This response selectivity is termed system selective coordination. To achieve such coordination the protective devices must be chosen on the basis of their particular response time-current characteristics so as to operate on the minimum current that will enable them to distinguish between fault currents and permissable, short term load currents in excess of rated current. Each device should operate in the minimum time possible and yet be selective, i.e., coordinate, with other devices in series therewith. When these two requirements are met, damage to the system and service interruption are minimized.
It is thus appreciated that the concerns of the adequate distribution system protection and coordination involved in the selection of the appropriate circuit protective devices to be implemented requires a complete understanding of the response time-current curves of the numerous types and kinds of circuit protective devices available. For it is on the basis of these curves that the circuit protective device selections are made pursuant to achieving requisite system protection and coordination on the most economically practical basis.
U.S. Pat. No. 4,266,259 to Howell also assigned to the assignee of the instant invention, describes a static trip circuit breaker which includes provision for zone selective interlocking functions. This patent is incorporated herein for purposes of reference. The patent is particularly useful in providing a detailed description of the operation of the circuit components to provide time-trip characteristics for various current magnitudes.